Mode Switcher, Heat Recovery Multi-Split Air Conditioning System and Control Method

ABSTRACT

A mode switcher includes at least one mode conversion branch. The mode conversion branch includes: a liquid pipe section, a high-pressure gas pipe section and a low-pressure gas pipe section. The mode switcher further includes a high-pressure electronic expansion valve and a low-pressure electronic expansion valve, the high-pressure electronic expansion valve and the low-pressure electronic expansion valve are respectively arranged in series in the high-pressure gas pipe section and the low-pressure gas pipe section, so as to gradually be opened and closed during air conditioning mode switching to reduce a differential pressure between before and after a mode conversion operation. A heat recovery multi-split air conditioning system and control method are further provided.

RELEVANT APPLICATION

The present application claims the priority of Chinese Application No. 201710811336.X, filed on Sep. 11, 2017 and entitled “MODE SWITCHER, HEAT RECOVERY MULTI-SPLIT AIR CONDITIONING SYSTEM AND CONTROL METHOD”, the entire contents of which are herein incorporated by reference.

TECHNICAL FIELD

The present application relates to the field of air conditioning, and in particular, to a mode switcher, a heat recovery multi-split air conditioning system and a control method.

BACKGROUND

A heat recovery multi-split air conditioning system is an air conditioning equipment, which can meet the simultaneous refrigeration and heating requirements of multiple rooms, and can convert the heat exchange energy of partial space into other spaces in the system for utilization so as to achieve the purpose of energy recovery, and reasonable transfer and utilization of energy in the air conditioning system are achieved.

In the heat recovery multi-split air conditioning system, a mode switcher is a very important functional component, it can not only replace a branch pipe, but also is responsible for a mode switching function of the unit running. In the existing mode switcher, a solenoid valve is mainly used to switch a liquid pipe and a high/low pressure gas pipe, due to the large differential pressure between the front and back of a valve body of the solenoid valve, when mode switching is performed, the noise at the moment of switching the valve body is very obvious. On the other hand, the sound of liquid flow is more obvious during the operation of the system. Therefore, how to effectively reduce the operation noise of the heat recovery multi-split air conditioning system has become a technical problem that needs to be solved urgently.

SUMMARY

One object of the present application is to provide a mode switcher and a control method, which can effectively reduce the noise when air conditioning mode is switched by the mode switcher.

Another object of the present application is to provide a heat recovery multi-split air conditioning system, which can effectively reduce the running noise of the heat recovery multi-split air conditioning system.

To achieve one of the foregoing objects, the present application provides a mode switcher, including at least one mode conversion branch, wherein the mode conversion branch includes: a liquid pipe section, a high-pressure gas pipe section and a low-pressure gas pipe section, both ends of the liquid pipe section are respectively used for connecting a main liquid pipe and a branch liquid pipe, one end of the high-pressure gas pipe section and one end of the low-pressure gas pipe section are respectively used for connecting a main high-pressure gas pipe and a main low-pressure gas pipe, another end of the high-pressure gas pipe section and another end of the low-pressure gas pipe section are both connected with a branch gas pipe, the mode switcher further includes a high-pressure electronic expansion valve and a low-pressure electronic expansion valve, the high-pressure electronic expansion valve and the low-pressure electronic expansion valve are respectively arranged in series in the high-pressure gas pipe section and the low-pressure gas pipe section, and are configured to gradually be opened and closed during air conditioning mode switching to reduce a differential pressure between before and after a mode conversion operation.

Further, the mode conversion branch further includes a high-pressure solenoid valve and a low-pressure solenoid valve, and the high-pressure solenoid valve and the low-pressure solenoid valve are respectively connected in parallel with the high-pressure electronic expansion valve and the low-pressure electronic expansion valve through bypass pipelines.

Further, the liquid pipe section is further connected with a supercooling pipe in series for performing heat exchange on a refrigerant of the liquid pipe section and a refrigerant led from the main liquid pipe to the main low-pressure gas pipe.

Further, the mode conversion branches are connected in parallel, in series, or in series-parallel form through the main liquid pipe, the main high-pressure gas pipe and the main low-pressure gas pipe.

In order to achieve one of the above objects, the present application further provides a heat recovery multi-split air conditioning system, including an outdoor unit, an indoor unit and the aforementioned mode switcher, wherein the outdoor unit is connected with various mode conversion branches in the mode switcher through a main liquid pipe, a main high-pressure gas pipe and a main low-pressure gas pipe, and the mode conversion branches are connected with a corresponding indoor unit through a branch liquid pipe and a branch gas pipe.

Further, muffler is arranged on at least one side of the main high-pressure gas pipe and the main low-pressure gas pipe connecting the mode switcher.

Further, the liquid pipe section is further connected with a supercooling pipe in series, the heat recovery multi-split air conditioning system further includes a first bypass pipeline led from the main liquid pipe to the main low-pressure gas pipe, the first bypass pipeline is connected with the supercooling pipe and performs heat exchange, and a filter and a supercooling throttling unit are arranged in series in the first bypass pipeline.

Further, the heat recovery multi-split air conditioning system further includes a second bypass pipeline arranged between the main high-pressure gas pipe and the main low-pressure gas pipe, and a gas bypass solenoid valve and a throttling unit are arranged in series in the second bypass pipeline.

In order to achieve one of the above objects, the present application further provides a control method based on the foregoing mode switcher, including:

upon receiving a switching instruction of switching from a cooling mode to a heating mode, controlling the low-pressure electronic expansion valve to gradually decrease the opening degree at a preset step size, until the low-pressure electronic expansion valve is closed; and then controlling the high-pressure electronic expansion valve to be opened, adjusting the high-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, controlling the high-pressure electronic expansion valve to gradually increase the opening degree at the preset step size, until the high-pressure electronic expansion valve is opened to a maximum opening degree; and/or

upon receiving a switching instruction of switching from the heating mode to the cooling mode, controlling the high-pressure electronic expansion valve to gradually decrease the opening degree at a preset step size, until the high-pressure electronic expansion valve is closed; and then controlling the low-pressure electronic expansion valve to be opened, adjusting the low-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, controlling the low-pressure electronic expansion valve to gradually increase the opening degree at the preset step size, until the low-pressure electronic expansion valve is opened to a maximum opening degree.

Further, the mode conversion branch further includes a high-pressure solenoid valve and a low-pressure solenoid valve, and the high-pressure solenoid valve and the low-pressure solenoid valve are respectively connected in parallel with the high-pressure electronic expansion valve and the low-pressure electronic expansion valve through bypass pipelines; and the control method further includes:

upon receiving the switching instruction of switching from the cooling mode to the heating mode, further controlling the low-pressure solenoid valve to be closed, and opening the high-pressure solenoid valve when controlling the high-pressure electronic expansion valve to be opened to the maximum opening degree; and

upon receiving the switching instruction of switching from the heating mode to the cooling mode, further controlling the high-pressure solenoid valve to be closed, and opening the low-pressure solenoid valve when controlling the low-pressure electronic expansion valve to be opened to the maximum opening degree.

Based on the above technical solutions, in the present application, the electronic expansion valves are arranged in series in the high-pressure gas pipe section and the low-pressure gas pipe section in the mode conversion branch of the mode switcher, and the differential pressure of the valve refrigerant between before and after the mode switching operation is reduced by using the gradual opening and closing of the electronic expansion valves during the air conditioning mode switching, thereby reducing the noise caused by the differential pressure at the moment of switching; and when the mode switcher of the present application is applied to the heat recovery multi-split air conditioning system, the running noise of the heat recovery multi-split air conditioning system can be effectively reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are used for providing a further understanding of the present application and constitute a part of the present application. Exemplary embodiments of the present application and illustrations thereof are used for explaining the present application, but constitute no limitation to the present application. In the drawings:

FIG. 1 is a structural schematic diagram of one embodiment of a heat recovery multi-split air conditioning system of the present application.

FIG. 2 is a structural schematic diagram of another embodiment of the heat recovery multi-split air conditioning system of the present application.

FIG. 3 is a schematic diagram of a flow direction of a refrigerant when a certain mode conversion branch is switched to a heating mode in an embodiment of a mode switcher of the present application.

FIG. 4 is a schematic diagram of a flow direction of a refrigerant when a certain mode conversion branch is switched to a cooling mode in an embodiment of a mode switcher of the present application.

FIG. 5 to FIG. 7 are respectively schematic diagrams of parallel, series and series-parallel connection forms of various mode conversion branches in an embodiment of a mode switcher of the present application.

DETAILED DESCRIPTION

The technical solution of the present application is described in further detail below with reference to the drawings and embodiments.

As shown in FIG. 1, it is a structural schematic diagram of one embodiment of a heat recovery multi-split air conditioning system of the present application. Referring to FIG. 5 at the same time, heat recovery multi-split air conditioning system of the present embodiment includes an outdoor unit O, an indoor unit and a mode switcher. The outdoor unit O is connected with various mode conversion branches B1, B2, . . . , BN in the mode switcher through a main liquid pipe P1, a main high-pressure gas pipe P3 and a main low-pressure gas pipe P2. The mode conversion branches B1, B2, . . . , BNare connected with the corresponding indoor unit (not shown in the figure) through a branch liquid pipe P4 and a branch gas pipe P5. The high pressure and low pressure are relative concepts therein, and their specific values can be selected according to the actual situation.

In the present embodiment, the various mode conversion branches can select corresponding air conditioning modes according to demands, if all mode conversion branches are switched to a cooling mode, it can be referred to as a complete cooling mode, if all mode conversion branches are switched to a heating mode, it can be referred to as a complete heating mode, if a part of mode conversion branches is switched to the cooling mode and the other part of mode conversion branches is switched to the heating mode, heat recovery can be achieved between both parts of mode conversion branches, and thus it can be referred to as a heat recovery mode.

In FIG. 1, the various mode conversion branches adopt a connection mode as shown in FIG. 6, that is, are connected in series through the main liquid pipe P1, the main high-pressure gas pipe P3 and the main low-pressure gas pipe P2. In other embodiments, the various mode conversion branches can also adopt a parallel connection mode as shown in FIG. 5, that is, the mode conversion branches P1, . . . , PN are connected in parallel through the main liquid pipe P1, the main high-pressure gas pipe P3 and the main low-pressure gas pipe P2, and this connection mode can make the various mode conversion branches independent of each other and not interfere with each other. FIG. 7 further shows another feasible connection mode of the mode conversion branches, that is, a series-parallel connection mode, the main liquid pipe P1, the main high-pressure gas pipe P3 and the main low-pressure gas pipe P2 led out from the outdoor unit O can be divided into multiple groups of parallel pipelines, each group of parallel pipelines is connected in series with multiple mode conversion branches, and this connection mode can make the mode conversion branches of various parallel pipelines independent of each other and not interfere with each other. For example, the mode conversion branches B11, . . . , B1N of the group one of parallel pipelines and the mode conversion branches BN1, . . . , BNN of the group N of parallel pipelines are independent of each other and do not interfere with each other. For designers, they can choose the appropriate connection mode of the mode conversion branches according to actual needs.

The composition and operation mechanism of a mode switcher embodiment provided in the present application are described below with reference to FIG. 1. The mode switcher of the present embodiment includes at least one mode conversion branch B1, B2, . . . , BN. The mode conversion branch includes: a liquid pipe section, a high-pressure gas pipe section and a low-pressure gas pipe section, both ends of the liquid pipe section are respectively used for connecting a main liquid pipe P1 and a branch liquid pipe P4, one end of the high-pressure gas pipe section and one end of the low-pressure gas pipe section are respectively used for connecting a main high-pressure gas pipe P3 and a main low-pressure gas pipe P2, and another end of the high-pressure gas pipe section and another end of the low-pressure gas pipe section are both connected with a branch gas pipe P5. The mode switcher further includes a high-pressure electronic expansion valve 3 and a low-pressure electronic expansion valve 2, the high-pressure electronic expansion valve 3 and the low-pressure electronic expansion valve 2 are respectively arranged in series in the high-pressure gas pipe section and the low-pressure gas pipe section.

In the existing mode switcher, the high-pressure gas pipe section and the low-pressure gas pipe section are provided with solenoid valves, when air conditioning mode switching is needed, the solenoid valves need to be opened and closed, and the differential pressure between the front and the back of the solenoid valve body is very large, so that the noise at the moment of switching is also very loud. In order to solve the problem of noise at the moment of switching, the present embodiment adopts a manner of respectively disposing the high-pressure electronic expansion valve and the low-pressure electronic expansion valve in the high-pressure gas pipe section and the low-pressure gas pipe section respectively, and adjusting the pressure change at the moment of air conditioning mode switching by means of the adjustability of the opening degrees of the electronic expansion valve. That is to say, during the air conditioning mode switching, the high-pressure electronic expansion valve and the low-pressure electronic expansion valve are gradually opened and closed to reduce the differential pressure between before and after the mode conversion operation.

For the mode switcher, upon receiving a switching instruction of switching from a cooling mode to a heating mode, the mode switcher can control the low-pressure electronic expansion valve 2 to gradually decrease the opening degree at a preset step size, until it is closed, in this way, the differential pressure change between the front and back of the valve body of the low-pressure electronic expansion valve 2 is reduced. Then, the mode switcher controls the high-pressure electronic expansion valve 3 to be opened, adjusts the high-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, controls the high-pressure electronic expansion valve 3 to gradually increase the opening degree at the preset step size, until it is opened to the maximum opening degree. By gradually increasing the opening degree to the maximum opening degree after maintaining the preset initial opening degree for the preset time length, the differential pressure change between the front and back of the valve body of the high-pressure electronic expansion valve 3 is reduced, in this way, by gradually controlling the opening and closing of the low-pressure electronic expansion valve 2 and the high-pressure electronic expansion valve 3 in a heating mode switching process, the noise problem caused by the relatively large differential pressure at the moment of switching the heating mode is reduced or avoided.

Similarly, upon receiving a switching instruction of switching from the heating mode to the cooling mode, the mode switcher controls the high-pressure electronic expansion valve 3 to gradually decrease the opening degree at a preset step size, until it is closed; and then controls the low-pressure electronic expansion valve 2 to be opened, adjusts the low-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, controls the low-pressure electronic expansion valve 2 to gradually increase the opening degree at the preset step size, until it is opened to a maximum opening degree. The noise problem caused by the relatively large differential pressure at the moment of switching the cooling mode is also solved.

As shown in FIG. 2, it is a structural schematic diagram of another embodiment of the heat recovery multi-split air conditioning system of the present application. Compared with the previous embodiment, the mode conversion branch in the present embodiment further includes a high-pressure solenoid valve 5 and a low-pressure solenoid valve 4. The high-pressure solenoid valve 5 and the low-pressure solenoid valve 4 are connected in parallel with the high-pressure electronic expansion valve 3 and the low-pressure electronic expansion valve 2 through bypass pipelines, respectively.

Upon receiving the switching instruction of switching from the cooling mode to the heating mode, the mode switcher further controls the low-pressure solenoid valve 4 to be closed, and opens the high-pressure solenoid valve 5 when controlling the high-pressure electronic expansion valve 3 to be opened to the maximum opening degree. That is to say, upon receiving the switching instruction of switching from the cooling mode to the heating mode, the mode switcher can control the low-pressure solenoid valve 4 to be closed, and control the low-pressure electronic expansion valve 2 to gradually decrease the opening degree at the preset step size, until it is closed. Then, the mode switcher controls the high-pressure electronic expansion valve 3 to be opened, adjusts the high-pressure electronic expansion valve to the preset initial opening degree, after maintaining the preset initial opening degree for the preset time length, controls the high-pressure electronic expansion valve 3 to gradually increase the opening degree at the preset step size, until it opens to the maximum opening degree, and then opens the high-pressure solenoid valve 5.

Upon receiving the switching instruction of switching from the heating mode to the cooling mode, the mode switcher further controls the high-pressure solenoid valve 5 to be closed, and opens the low-pressure solenoid valve 4 upon controlling the low-pressure electronic expansion valve 2 to be opened to the maximum opening degree. That is to say, upon receiving the switching instruction of switching from the heating mode to the cooling mode, the mode switcher can control the high-pressure solenoid valve 5 to be closed, and control the high-pressure electronic expansion valve 3 to gradually decrease the opening degree at the preset step size, until it is closed. Then, the mode switcher controls the low-pressure electronic expansion valve 2 to be opened, adjusts the low-pressure electronic expansion valve to the preset initial opening degree, after maintaining the preset initial opening degree for the preset time length, controls the low-pressure electronic expansion valve 2 to gradually increase the opening degree at the preset step size, until it is opened to the maximum opening degree, and then opens the low-pressure solenoid valve 4.

For an existing mode sensor, the present embodiment can be obtained by connecting electronic expansion valves in parallel to the solenoid valves in a high-pressure gas path section and a low-pressure gas path section, so it is convenient for reconstruction and is easy to implement.

In the mode conversion branches of the aforementioned two mode switchers, a supercooling pipe 6 can also be connected in series to the liquid pipe section. The supercooling pipe 6 can perform heat exchange on a refrigerant of the liquid pipe section and a refrigerant led from the main liquid pipe P1 to the main low-pressure gas pipe P2, so as to maintain the stability of the degree of supercooling of the discharge liquid of the liquid pipe, and then running of the electronic expansion valves is more stable, and the comfort of the air conditioning system is improved. In addition, the supercooling pipe 6 can further reduce the degree of supercooling of the refrigerant, and can improve the heat exchange effect of the indoor unit within a certain extent. Correspondingly, the heat recovery multi-split air conditioning system of the present application further includes a first bypass pipeline led from the main liquid pipe P1 to the main low-pressure gas pipe P2. The first bypass pipeline is connected with the supercooling pipe 6 and performs heat exchange with the supercooling pipe 6, and a filter 7 and a supercooling throttling unit 8 are arranged in series in the first bypass pipeline. The supercooling throttling unit 8 is used for reducing pressure of the refrigerant from the main liquid pipe P1 by regulating flow, and the filter 7 can filter impurities in the refrigerant from the main liquid pipe P1. In conjunction with supercooling control, a supercooling gas intake temperature sensing pack 11 can also be arranged at the downstream of the supercooling throttling unit 8, and a supercooling gas exhaust temperature sensing pack 12 is arranged at a tail section of the first bypass pipeline, so that the supercooling throttling unit 8 controls the opening degree according to the sensing data of the supercooling gas intake temperature sensing pack 11 and the supercooling gas exhaust temperature sensing pack 12. In addition, the heat recovery multi-split air conditioning system can further include a second bypass pipeline arranged between the main high-pressure gas pipe P3 and the main low-pressure gas pipe P2, and a gas bypass solenoid valve 9 and a throttling unit 10 are arranged in series in the second bypass pipeline. The bypass pipeline, and the throttling unit 10 and the gas bypass solenoid valve 9 therein can play a role of unloading when the system high-pressure is too high, that is, when the system high-pressure exceeds a predetermined value, the gas bypass solenoid valve 9 is opened to transport the refrigerant on a high pressure side to a low pressure side through the second bypass pipeline so as to reduce the system pressure on the high pressure side.

As above mentioned in the description of the embodiment of the mode switcher, the noise at the moment of switching can be effectively reduced or eliminated by controlling the gradual opening and closing of the electronic expansion valve during the air conditioning mode switching, in consideration that the sound of liquid flow during running of the heat recovery multi-split air conditioning system is also obvious, therefore, mufflers 1 can be added in the embodiment of the heat recovery multi-split air conditioning system, referring to FIG. 1 and FIG. 2, the mufflers 1 can be arranged on at least one side of the main high-pressure gas pipe P3 and at least one side of the main low-pressure gas pipe P2. In this way, the sound of liquid flow during running of the system is suppressed, and in combination with the control of the gradual opening and closing of the electronic expansion valve, the running noise of the heat recovery multi-split air conditioning system is effectively reduced.

On the basis of the foregoing mode switcher embodiment of the present application, the present application further provides a control method based on the foregoing mode switcher, including:

upon receiving a switching instruction of switching from a cooling mode to a heating mode, controlling the low-pressure electronic expansion valve 2 to gradually decrease the opening degree at a preset step size, until the low-pressure electronic expansion valve 2 is closed; and then controlling the high-pressure electronic expansion valve 3 to be opened, adjusting the high-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, controlling the high-pressure electronic expansion valve 3 to gradually increase the opening degree at the preset step size, until the high-pressure electronic expansion valve 3 is opened to a maximum opening degree; and/or

upon receiving a switching instruction of switching from the heating mode to the cooling mode, controlling the high-pressure electronic expansion valve 3 to gradually decrease the opening degree at a preset step size, until the high-pressure electronic expansion valve 3 is closed; and then controlling the low-pressure electronic expansion valve 2 to be opened, adjusting the low-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, controlling the low-pressure electronic expansion valve 2 to gradually increase the opening degree at the preset step size, until the low-pressure electronic expansion valve 2 is opened to a maximum opening degree.

In the embodiment of another mode switcher, the mode conversion branch can further include a high-pressure solenoid valve 5 and a low-pressure solenoid valve 4, and the high-pressure solenoid valve 5 and the low-pressure solenoid valve 4 are respectively connected in parallel with the high-pressure electronic expansion valve 3 and the low-pressure electronic expansion valve 2 through bypass pipelines. Correspondingly, the control method further includes:

upon receiving the switching instruction of switching from the cooling mode to the heating mode, further controlling the low-pressure solenoid valve (4) to be closed, and opening the high-pressure solenoid valve (5) when controlling the high-pressure electronic expansion valve (3) to be opened to the maximum opening degree; and

upon receiving the switching instruction of switching from the heating mode to the cooling mode, further controlling the high-pressure solenoid valve (5) to be closed, and opening the low-pressure solenoid valve (4) when controlling the low-pressure electronic expansion valve (2) to be opened to the maximum opening degree.

Different switching processes of a certain mode conversion branch of the mode switcher in the embodiment as shown in FIG. 2 are illustrated below in combination with FIG. 3 and FIG. 4.

The state shown in FIG. 3 is a refrigerant running state after switching to the heating mode, and the state shown in FIG. 4 is the refrigerant operation state after switching to the cooling mode. Solid lines in FIG. 3 and FIG. 4 indicate the flow directions of the refrigerant in the liquid pipes, and broken lines indicate the flow direction of the refrigerant in the gas pipes. It is assumed that the heating mode shown in FIG. 3 is an initial state, when the mode switcher receives the switching instruction of switching the mode conversion branch to the cooling mode, the high-pressure solenoid valve of a high-pressure gas path section is controlled to close, and the high-pressure electronic expansion valve is controlled to gradually decrease the opening degree at the preset step size (for example, the opening degree is decreased at the step size P1 every S1 seconds), until it is closed. Then, the low-pressure electronic expansion valve of a low-pressure gas path section is controlled to open, the low-pressure electronic expansion valve is adjusted to the preset initial opening degree P0. After maintaining the preset initial opening degree P0 of the low-pressure electronic expansion valve for the preset time length S2, the low-pressure electronic expansion valve is controlled to gradually increase the opening degree at the preset step size (for example, the opening degree is increased at the step size P2 every S3 seconds), until it is opened to the maximum opening degree Pmax, and then the low-pressure solenoid valve is opened. At this time, the refrigerant cycle has been switched to the cooling mode as shown in FIG. 4. In the switching process, through the control of the solenoid valves and the electronic expansion valves, the original instantaneous pressure change process is converted into a relatively long time slow change, so the noise source of the instantaneous differential pressure is fundamentally eliminated, thereby achieving a good denoising effect. Similarly, the control process in which the cooling mode as shown in FIG. 4 is the initial state and then is switched to the heating mode as shown in FIG. 3 can refer to the foregoing description, and thus is not repeated herein again.

Finally, it should be noted that the above-mentioned embodiments are merely used for illustrating the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they could still make modifications to specific embodiments of the present application or make equivalent substitutions to a part of technical features, without departing from the spirit of the technical solutions of the present application, and these modifications or substitutions shall all fall within the scope of the technical solutions of the present application. 

1. A mode switcher, comprising at least one mode conversion branch, wherein the at least one mode conversion branch comprises: a liquid pipe section, a high-pressure gas pipe section and a low-pressure gas pipe section, both ends of the liquid pipe section are respectively configured to connect a main liquid pipe and a branch liquid pipe, one end of the high-pressure gas pipe section and one end of the low-pressure gas pipe section are respectively configured to connect a main high-pressure gas pipe and a main low-pressure gas pipe, another end of the high-pressure gas pipe section and another end of the low-pressure gas pipe section are configured to both connect with a branch gas pipe, the mode switcher further comprises a high-pressure electronic expansion valve and a low-pressure electronic expansion valve, the high-pressure electronic expansion valve and the low-pressure electronic expansion valve are respectively arranged in series in the high-pressure gas pipe section and the low-pressure gas pipe section, so as to gradually be opened and closed during air conditioning mode switching to reduce a differential pressure between before and after a mode conversion operation.
 2. The mode switcher according to claim 1, wherein the at least one mode conversion branch further comprises a high-pressure solenoid valve and a low-pressure solenoid valve, and the high-pressure solenoid valve and the low-pressure solenoid valve are respectively connected in parallel with the high-pressure electronic expansion valve and the low-pressure electronic expansion valve through bypass pipelines.
 3. The mode switcher according to claim 1, wherein the liquid pipe section is further connected with a supercooling pipe in series so as to perform heat exchange on a refrigerant of the liquid pipe section and a refrigerant led from the main liquid pipe to the main low-pressure gas pipe.
 4. The heat recovery multi-split air conditioning system according to claim 1, wherein all mode conversion branches are connected in parallel, in series, or in series-parallel form through the main liquid pipe, the main high-pressure gas pipe and the main low-pressure gas pipe.
 5. A heat recovery multi-split air conditioning system, comprising an outdoor unit, an indoor unit and the mode switcher of claim 1, wherein the outdoor unit is connected with various mode conversion branches in the mode switcher through a main liquid pipe, a main high-pressure gas pipe and a main low-pressure gas pipe, and the mode conversion branches are connected with a corresponding indoor unit through a branch liquid pipe and a branch gas pipe.
 6. The heat recovery multi-split air conditioning system according to claim 5, wherein a muffler is arranged on at least one of the main high-pressure gas pipe and the main low-pressure gas pipe connecting the mode switcher.
 7. The heat recovery multi-split air conditioning system according to claim 5, wherein the liquid pipe section is further connected with a supercooling pipe in series, the heat recovery multi-split air conditioning system further comprises a first bypass pipeline led from the main liquid pipe to the main low-pressure gas pipe, the first bypass pipeline is connected with the supercooling pipe and performs heat exchange, and a filter and a supercooling throttling unit are arranged in series in the first bypass pipeline.
 8. The heat recovery multi-split air conditioning system according to claim 5, further comprising a second bypass pipeline arranged between the main high-pressure gas pipe and the main low-pressure gas pipe, and wherein a gas bypass solenoid valve and a throttling unit are arranged in series in the second bypass pipeline.
 9. A control method of the mode switcher of claim 1, comprising at least one of the following steps: upon receiving a switching instruction of switching from a cooling mode to a heating mode, controlling the low-pressure electronic expansion valve to gradually decrease an opening degree at a preset step size, until the low-pressure electronic expansion valve is closed; and then controlling the high-pressure electronic expansion valve to be opened, adjusting the high-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, causing the high-pressure electronic expansion valve to gradually increase the opening degree at the preset step size, until the high-pressure electronic expansion valve is opened to a maximum opening degree; upon receiving a switching instruction of switching from the heating mode to the cooling mode, causing the high-pressure electronic expansion valve to gradually decrease the opening degree at a preset step size, until the high-pressure electronic expansion valve is closed; and then causing the low-pressure electronic expansion valve to be opened, adjusting the low-pressure electronic expansion valve to a preset initial opening degree, and after maintaining the preset initial opening degree for a preset time length, causing the low-pressure electronic expansion valve to gradually increase the opening degree at the preset step size, until the low-pressure electronic expansion valve is opened to a maximum opening degree.
 10. The control method according to claim 9, wherein the at least one mode conversion branch further comprises a high-pressure solenoid valve and a low-pressure solenoid valve, and the high-pressure solenoid valve and the low-pressure solenoid valve are respectively connected in parallel with the high-pressure electronic expansion valve and the low-pressure electronic expansion valve through bypass pipelines; and the control method further comprises: upon receiving the switching instruction of switching from the cooling mode to the heating mode, causing the low-pressure solenoid valve to be closed, and opening the high-pressure solenoid valve when causing the high-pressure electronic expansion valve to be opened to the maximum opening degree; or upon receiving the switching instruction of switching from the heating mode to the cooling mode, causing the high-pressure solenoid valve to be closed, and opening the low-pressure solenoid valve when causing the low-pressure electronic expansion valve to be opened to the maximum opening degree. 